[Plugin: WordPress SEO by Yoast] Google Webmaster shows 404 Error for sitemap, but sitemap_index is

Suddenly Webmaster started showing 404 page not found for the sitemap. so i checked my sitemap_index.xml, and indeed http://www.mydomain.com/sitemap_index.xml was not found. so i did a google research and found out the problem was something to do with Yoast plugin. then i read somewhr here that u need to paste some code in the .htacess file. So i did that and now i can see the sitemap at http://www.mydomain.com/sitemap_index.xml.
But now the problem is when i resubmit the Sitemap to google webmaster. it still says the same error: 404 page not found error.. i searched all the forum but couldnt find any solution..
i would highly appreciate if someone could help me.. Thanks

http://wordpress.org/extend/plugins/wordpress-seo/